In communications systems, in particular, for example, radio communications systems, information (for example voice, picture information or other data) is transmitted by means of electromagnetic waves via a radio interface between a transmitting and a receiving station (base station and subscriber station). The electromagnetic waves are in this case transmitted at carrier frequencies which are in the frequency band provided for the respective system. By way of example, frequencies in the frequency band around 2000 MHz have been provided for the UMTS (Universal Mobile Telecommunications System) or other 3rd generation systems, for future mobile radio systems using, for example CDMA or TD/CMDA transmission methods (TD/CDMA: Time/Code Division Multiple Access) via the radio interface.
Individual system devices for mobile radio systems which are considered by way of example, such as base stations, require a high-precision clock source, particularly for radio operation. Oven-stabilized crystal oscillators are currently used as clock sources for this purpose, and are synchronized via a landline to a high-precision reference clock source (which is preferably defined with atomic accuracy) in a synchronous switching system, for example a PCM switching system (PCM: Pulse Code Modulation). Owing to the complexity of their production, these clock sources are expensive.
However, in the future, base stations will no longer be connected just to PCM networks but also to packet-switching networks (IP, ATM/asynchronous transfer mode). Such synchronization of clock sources via landlines will no longer be feasible, without problems. Other synchronization methods via satellite links (GPS: Global Positioning System) or via an atomic standard that is installed are feasible, but are generally very complex and expensive.
Another example is digital electronic selection systems, in which individual stations and devices likewise require complete synchronization. However, these are also subject to the problem that it is complex and expensive to supply a basic clock for tuning purposes, for the reasons mentioned above.
A cellular telecommunications network is known from WO 99 33207, which comprises a higher-level timing unit in a monitoring node in a network, and a lower-level timing unit in a monitoring node or a monitored node, with the timing unit which initiates the synchronization using the response of the synchronization analysis to determine the synchronization matching for the lower-level timing unit.
EP-A-0 283 079 describes an asynchronous time domain system, which contains at least one node with a switching unit to which a large number of user stations are connected via transmission connections, and which is suitable for connecting the user stations to one another, with at least one of the user stations being a timing station, whose time information is made available to the other user stations once the connections have been set up.